The invention relates to the field of digital discs, and, more particularly, to compact disks (CDs) such as Read Only Memory (ROM) disks and digital versatile disks (DVDs).
The invention applies advantageously but not limitingly to the field of digital disks, CD-ROMs and, more particularly, to DVDs which store image data in a compressed manner. Furthermore, the invention pertains to the calculation of the time interval between successive transitions, with respect to a threshold, of a sampled incident signal.
The invention makes it possible in particular to detect a phase shift between two elementary signals emanating from photodetectors. The phase shift corresponds to a positioning error of the incident optical beam (e.g., a laser spot) illuminating a track of the disk. This positioning error is then used conventionally in a slaving loop to modify the incident optical system and to bring the optical beam back to the track and slave it thereto.
The determination of the time interval between two successive transitions of a sampled incident signal conventionally requires the use of several memories of the first-in first-out (FIFO) type to store in succession a number of samples of the signal received. Furthermore, a relatively complex management system is also required for tagging occurrences of the transitions.
The invention provides a hardware solution for calculating a time interval between two selected successive transitions of a sampled incident signal with respect to a threshold. More specifically, an electronic device is provided for calculating the time interval between two selected successive transitions of a sampled incident signal with respect to a threshold.
According to the invention, the device includes a detection and selection circuit or means able to detect and to select transitions of the incident signal. The device according to the invention makes it possible to relatively easily calculate the time interval between two successive transitions regardless of the direction of these transitions. That is, whether the signal oversteps the threshold in one direction or in the other. This being so, the detection and selection means can select rising or falling transitions of the incident signal to determine the time interval between two transitions of like direction.
The device according to the invention may further include a calculating circuit or means for calculating a time gap or phase between each selected transition and an edge of a sampling clock signal. The device may also include at least one unit or assemblage of at least three blocks, each block including a storage cell and a temporal counter regulated by the sampling clock signal. A checking circuit or means may also be provide which is able, in response to each selected current transition, to trigger an activation processing of the assemblage.
The activation processing may include initializing the temporal counter of the first block of the assemblage, storing the value of the time gap (or phase) corresponding to the selected current transition in the storage cell of the first block, and transferring the content of a block of the assemblage into a next block. Processing means are then able to determine the time interval between two successive selected transitions based upon the values of the temporal counters of the second and third blocks and of the values of the time gaps (or phases) respectively stored in the cells of the second and third blocks.
In an application for determining a mutual phase shift between two elementary signals, the device may advantageously include two assemblages of at least three blocks. Each assemblage may be respectively dedicated to transitions of opposite directions, for example. A multiplexing means or multiplexer controlled as a function of the sign of each transition detected may also be included. The successive transitions having the same direction, whether they originate from one or the other of the two elementary signals, may be processed in one of the assemblages of three blocks. On the other hand, the successive transitions having the other direction, whether they originate from one or the other of the two elementary signals, may be processed in the second assemblage.
According to the invention, a digital disk reader, such as a DVD disk reader, for example, incorporating a device as described above is also provided.